Voltage regulators, which are a type of power converter, provide an output voltage to a load within a desired range of a nominal regulated value from a voltage source that may be poorly-specified or fluctuating, or that may be at an inappropriate amplitude for the load. Such regulators may employ a switching circuit that includes one or more switching elements coupled in series or in parallel with the load. The switching elements may be, for example, power metal-oxide semiconductor field-effect transistor (MOSFET) switches.
Control circuitry regulates the output voltage and the current supplied to the load by cycling the switch circuit between ON and OFF states. The duty cycle of the switch circuit controls the flow of power to the load, and can be varied by a variety of methods. For example, the duty cycle can be varied by (1) fixing the pulse stream frequency and varying the ON or OFF time of each pulse, (2) fixing the ON or OFF time of each pulse and varying the pulse stream frequency, or (3) a combination thereof.
To vary the ON or OFF time of each pulse or the pulse stream frequency, the control circuitry may generate a signal Ve that is proportional to the difference between the regulator's output voltage and a reference voltage. Ve may be used to provide either “voltage-mode” or “current-mode” regulation. In voltage-mode regulation, Ve and a periodic sawtooth waveform Vs may be provided as inputs to a comparator, the output of which controls the duty cycle of the switch circuit. In current-mode regulation, a voltage Vi may be generated that is proportional to the current in the output inductor, and Vi and Ve may be provided as inputs to a comparator, the output of which controls the duty cycle of the switch circuit.
Synchronous switching regulators include at least two active switching elements that typically are driven by non-overlapping drive signals to supply current at an output voltage to a load within a desired range of a nominal regulated value. Synchronous switching regulators that use power MOSFET switches frequently are used in portable battery-powered electronic products and thermally-sensitive products. These regulators convert the typically fluctuating input voltage to a regulated output voltage. Such regulators can provide high operating efficiency and thus long battery life with little heat generation.
One fault condition that a regulator may experience is an over-current condition at the regulator output, where the current demanded by the load is significantly greater than the nominal maximum output current of the regulator. The over-current condition may cause excessive currents to flow through the components of the regulator and to be delivered at the regulator's output, causing potential damage to those components or the load, particularly when the over-current condition remains at the regulator output for a prolonged period of time.
Previous designs of switching regulators, such as the LTC1702 synchronous voltage-mode controlled buck regulator, have implemented current limit protection by comparing the drain-to-source voltage (VDS) of one of the active switch elements, e.g., a MOSFET, of a synchronous switch to a reference voltage set by the user that represents the maximum allowable VDS voltage. The VDS voltage provides inductor current information through the relationship:VDS=IL*RDS(ON)  EQ. 1where IL is the inductor current and RDS(ON) is the resistance of the MOSFET when the MOSFET is ON. Since average inductor current IL,AVG approximately equals the output current in a buck regulator, the inductor current flowing through the MOSFET can be used as an indicator of the output current.
The LTC1702 compares the VDS voltage of the MOSFET to the user-set reference voltage with a transconductance (gm) amplifier, the output of which, averaged by an external capacitor, controls the duty cycle of the switching regulator. When the current limit is exceeded, the duty cycle is reduced slowly until the output current is regulated at the programmed current limit. One of the problems with this approach is that there may be a delay in reducing the initial duty cycle to the lower duty cycle. During the transient phase when the duty cycle is being reduced, the inductor current is unregulated and may cause excessive currents to flow, limited only by the normally small input source, MOSFET and inductor impedances.
Other current limit schemes, such as that employed by the TPS40050 buck regulator, uses a cycle-by-cycle comparator to monitor the VDS voltage of a top-side MOSFET of a synchronous switch and to instantly turn OFF the MOSFET when the VDS exceeds a maximum allowable voltage. This scheme, however, also has drawbacks because the top MOSFET must be turned ON to sense the current flowing through the inductor—a situation that is undesirable during an over-current condition—and thus requires an additional fault counter and restart scheme to keep the inductor current from running away, all without ever achieving steady-state regulation of the output current (i.e., regulation of the average value of the output current).
In view of the foregoing, it would be desirable to provide methods and circuits for protecting power converters from over-current conditions by providing both steady-state and cycle-by-cycle current limit protection.
It also would be desirable to provide methods and circuits for protecting power converters from over-current conditions, in which current limit thresholds are user-programmable.
It further would be desirable to provide methods and circuits for protecting power converters from over-current conditions, in which a signal indicative of inductor current flowing through the power converter is obtained without increasing output current, and thereby exacerbating the over-current condition.
It still further would be desirable to provide methods and circuits for protecting power converters from over-current conditions by providing cycle-by-cycle current limit protection, in which a signal indicative of inductor current flowing through the power converter is obtained without increasing output current, and thereby exacerbating the over-current condition.